1. Field of the Invention
Exemplary aspects of the present invention relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing a toner image on a recording medium, and an image forming apparatus including the fixing device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charging device uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
Such a fixing device may include an endless fixing belt serving as a fixing member formed into a loop, a heater provided inside the loop to heat the fixing belt, and a pressing roller pressing the fixing belt to form a fixing nip therebetween. As a recording medium passes through the fixing nip, the fixing member applies heat to the recording medium to melt the toner image and fix it onto the recording medium.
Generally, fixing devices need to accommodate different sizes of recording media sheets. For example, a recording medium having a width narrower than a heating area of the fixing member in the axial direction thereof may be fed to the fixing nip. In such a case, a portion of the fixing member, for example, end portions in the axial direction, which does not contact the recording medium, remains heated by the heating member, thereby getting overheated because there is no recording medium to draw heat from the fixing member at that portion.
In order to accommodate different sizes of recording media sheets, one related-art fixing device employs, for example, a heating roller including a plurality of heat sources having different distributions of heat generation in the width direction of a recording medium. The heat sources include, for example, a halogen heater, a sheet heat generating member, an electromagnetic induction heater, and so forth.
In this configuration, electric power is supplied only to the heat source(s) corresponding to the width of the recording medium, thereby heating only that portion of the heating roller corresponding to the width of the recording medium. Accordingly, the temperature of the end portion of the heating roller which the recording medium does not contact is prevented from getting overheated.
Although advantageous, a range of adjustment of the heating width is limited to the number of heat sources employed in the heating roller. In other words, the type of recording media sheets that the fixing device can accommodate depends on the number of the heat sources in the heating roller.
In view of the above, there is demand for a fixing device capable of accommodating different sizes of recording media sheets while at the same time preventing overheating of the fixing member.